1. Field of the Invention
The present invention relates to a method for forming ceramics, particularly ceramic coatings, having excellent properties such as heat resistance, abrasion resistance, gas barrier properties and corrosion resistance, at low temperature, by utilizing a polysilazane as an essential component. Further, it relates to a composition for forming such ceramic coatings at low temperature, and to an article which cannot be subjected to high temperature coated with SiO.sub.2 derived from a polysilazane by a method of the invention.
2. Description of Related Art
For properties such as high abrasion resistance, corrosion resistance, heat resistance and impermeability to water vapor, Na, and the like, organic paints are inadequate and ceramics based coatings are used.
Prior arts for forming ceramic coatings include PVD (e.g. sputtering), CVD, sol-gel process, polytitanocarbosilane paints, poly-(disil)silazane paints, polysilazane paints, polymetalosilazane paints, and the like.
These ceramic coatings have drawbacks. Specifically, PVD and CVD processes require expensive devices. In sol-gel process, firing temperatures of 500.degree. C. or higher are needed. Polytitanocarbosilane paints have insufficient surface strength when fired at low temperature (400.degree. C. or below). Poly(disil)silazane polymer is difficult to apply and is liable to yield cracks. Polysilazane and polymetalosilazane may be fired at a temperature of 200 to 500.degree. C. into coatings, but firing at a temperature below 300.degree. C. may lead to unacceptable coating quality.
Methods for coating surfaces are disclosed to provide plastic films with gas-barrier properties, hardness and heat resistance without impairing transparency of the film. For example, Japanese Unexamined Patent Publication (Kokai) No. 4-80030/1992 discloses a method for forming a combined inorganic/organic coating on a surface by coating a solution of alkoxysilane using a sol-gel process. Japanese Unexamined Patent Publication (Kokai) No. 6-93120/1994 discloses a method for forming SiO.sub.2 coatings on a base film having high gas barrier properties by a dry plating process such as vacuum deposition, ion plating and sputtering.
The above method as in Japanese Unexamined Patent Publication (Kokai) No. 4-80030/1992, however, requires a high temperature in forming coatings which limits the types of usable bases, results in low density coatings, and produces coatings having insufficient mechanical strength. Also, coatings prepared by the above method as in Japanese Unexamined Patent Publication (Kokai) No. 6-93120/1994, are expensive, have insufficient gas barrier properties, and suffer from cracks when producing a thick coating.
Accordingly, the main object of the present invention is to solve the above-mentioned problems in the prior art and provide a method for forming a ceramic coating which has good heat resistance, abrasion resistance, gas barrier properties and corrosion resistance, and is dense without cracks, by firing at low temperature. Particularly, by utilizing the characteristic low temperature firing, the specific purpose of the present invention is to make it possible to coat materials having low heat resistance such as electronic parts, for example, glass substrates for liquid crystals, color filters, printed circuit boards, IC and LSI boards; plastic products, for example, plastic films, resin glass, plastic plates and vinyl chloride pipes; wooden products; paper products; products plated with gold, silver, and the like; which materials cannot be coated by methods of the prior art; and semiconductor base materials (including silicon boards with wiring).